CODEWITHSUNDEEP
javaiteration
user3000019
user3000019

Reputation: 103

Reverse the order of accessing a list in a Java for loop

I want to reverse the order of accessing a List inside a for statement.

This is my actual code:

for(int i = 0; i < states.size(); i++) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the first blockstate
    states.remove(i); // remove the first blockstate from the list
}

This code works, but I would like to reverse it. I already tried other ways, like using i-- but it did not work. Can someone provide a suggestion?

Upvotes: 2

Views: 139

Answers (3)

M. Justin
M. Justin

Reputation: 21347

for (ListIterator<State> i = states.listIterator(states.size()); i.hasPrevious(); ) {
    State value = i.previous();

    System.out.println(states.size());
    value.update(true); // restore the first blockstate
    i.remove(); // remove the first blockstate from the list
}

Your stated desired code iterates over each element of the list in reverse order, and does the following things for each element it accesses:

  1. Print the remaining number of elements in the list
  2. Call the update method on the current element
  3. Remove the current element from the list

The ListIterator class allows iterating over the list in reverse order, accessing each element, and removing the current element from a list. It can therefore be used to achieve your stated goals.

Upvotes: 0

Pankaj Saboo
Pankaj Saboo

Reputation: 1185

Try this:

List list = Lists.reverse(states); 
for(int i = 0; i < list.size(); i++) {
    System.out.println(list.size());
    list.get(i).update(true); // restore the first blockstate
    //list.remove(i); // remove the first blockstate from the list
 }

if u want to remove element when accessing element then u have to use iterator by using simple for it is not possible... it throws concurrent modification exception

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 727047

I already tried other ways, like using i-- but it did not work.

Reversing a for loop consists of three steps:

  • Change the initial value to the last value,
  • Change the condition to stop after passing the initial value
  • Reversing the increment/decrement (i.e. ++ becomes --)

In your code this change would look as follows:

for(int i = states.size()-1 ; i >= 0 ; i--) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the current blockstate
    states.remove(i); // remove the current blockstate from the list
}

Note that the last value reached in the original loop is states.size()-1, not states.size(), so i needs to start at states.size()-1 as well.

Since your loop eventually clears out the list anyway, but does it one element at a time, you may get better clarity by deleting the call of remove(i), replacing it with states.clear() after the loop.

for(int i = states.size()-1 ; i >= 0 ; i--) {
    System.out.println(states.size());
    states.get(i).update(true); // restore the current blockstate
}
states.clear();

Upvotes: 6

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